Summary of Significant Accounting Policies (Earnings Per Share) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Computation of earnings per share
Net earnings (loss)	$ 38,499		$ 18,126		$ 81,016		$ 33,898		$ (414,733)		$ 134,001		$ 105,166		$ (52,497)		$ 171,539		$ (228,063)		$ 82,524
Basic (loss) earnings per share (in dollars per share)	$ 0.70	[1],[2],[3]	$ 0.33	[1],[2]	$ 1.47	[1],[2]	$ 0.62	[1],[2]	$ (7.62)	[1],[2],[4]	$ 2.45	[1],[2]	$ 1.93	[1],[2]	$ (0.98)	[1],[2]	$ 3.11	[1],[2]	$ (4.21)	[1],[2]	$ 1.54
Dilutive (loss) earings per share (in dollars per share)	$ 0.69	[1],[2],[3]	$ 0.33	[1],[2]	$ 1.46	[1],[2]	$ 0.61	[1],[2]	$ (7.62)	[1],[2],[4]	$ 2.42	[1],[2]	$ 1.90	[1],[2]	$ (0.98)	[1],[2]	$ 3.09	[1],[2]	$ (4.21)	[1],[2]	$ 1.52
Basic weighted average shares (in shares)																	54,912,000		54,133,000		52,969,000
Add: Dilutive effects of stock options (in shares)																	375,000		0		460,000
Diluted weighted average shares (in shares)																	55,287,000		54,133,000		53,429,000
Less: earnings allocable to participating securities																	885		0		1,199
Net earnings from available for common shareholders																	$ 170,654				$ 81,325
Options not included in the diluted earnings (loss) per share calculation (in shares)																	600,000		1,500,000		700,000

[1] (4)The sum of the individual quarterly net earnings (loss) per common share amounts may not agree with year-to-date net earnings (loss) per common share as each quarterly computation is based on the weighted average number of common shares outstanding during that period. Potentially dilutive securities were included in the computation of diluted net earnings (loss) per common share for each quarter in which the Company reported net earnings.

[2] (3)Due to the volatility of commodity prices, the estimated fair value of the Company's commodity derivative instruments is subject to fluctuations. As a result, the Company may recognize significant unrealized gains and losses (non-cash changes in fair value) on commodity derivative instruments from period to period.

[3] (1)In the fourth quarter of 2012, the Company recorded a $7.2 million income tax benefit for research and development tax credits

[4] (2)In the fourth quarter of 2011, the Company recorded an impairment of $625.0 million related to its E. Texas natural gas assets, largely due to the impact of lower natural gas prices.